"A difference that makes no difference is no difference at all." This neat little sentence is one of many neat little sentences William James (I fondly call him "Wild Bill"), came up with while rampaging through philosophy and psychology. He was a wordsmith, and also a great teacher, so it's said, and a popularizer (if there is such a word) of pragmatism and other things. Not the most precise thinker, though, and that prompted C.S. Peirce to call his philosophy "pragmaticism" to distinguish it from that of James. It also prompted, if it did not require, John Dewey to step in to tone down James' exuberance in response to criticism by such as Bertrand Russell. Being a Lord, and British, Russell had no tolerance for exuberance of any kind.
But though rambunctious, James could be most insightful, and one of his insights was that purported differences, some of which have occupied great minds far too much, make no difference. That is to say they do not result in any change to how we live. They don't satisfy any desire or urge, they don't serve to solve any question or problem we encounter in living, they don't impact what we do, how we react. They accomplish nothing, and cannot accomplish anything, unless perverted.
The perversion can result when a difference which makes no difference in itself is accepted as the fanciful grounds for or basis of a system or unwarranted inferences. For example, because we can't know what the world really is, and what it seems to be to us is all that we can know and perhaps even all that really is, the world is ours to make or shape; we are the masters of the world and may do with it what we will. This perversion is similar in some respects to religious beliefs; it breeds a kind of righteousness as it is founded on something which cannot be challenged.
Unfortunately, we persist in considering that which makes no difference, practically speaking, to be of greater worth than that which does. Too many feel truth and what is good are founded in something having an existence separate from the world, the world being fundamentally flawed. A holdover from Plato, perhaps, or religious beliefs. Whether or not there are ideal Forms or a transcendent divinity or absolute spirit, the world will be what it is and we will live in it as it is and we will be as we are, and so such creations of thought and belief make no difference themselves. But some of us take such speculations and impose them on ourselves and others
Differences which make no difference seem peculiar to philosophy and religion. Law has its share of fictions, but those fictions serve a purpose or at least make a difference in the sense that they must be treated or used in a certain fashion, may do certain things and not others. It may be said that philosophy seeks to explain life or aspects of living, and in that sense is not concerned necessarily in making a difference to how we live. That in itself is a noble and significant enterprise. The function of explanation, though, is being preempted more and more by science, and given the fact that philosophical explanations never seem to be accepted by most philosophers, even, it is unclear that explanations will ever be forthcoming from philosophers.
Making a difference is significant because the failure to make a difference indicates that what is being touted as a difference is at best trivial and of no account, at worst subject to perversion or malicious manipulation. Thus, while it may be the case that what we perceive as X is not what Kant would call the X in itself, we cannot know and have no reason to care whether it is or is not. We will keep on perceiving X as we perceive X and will do or not do what we always have and always will do or not do regarding X.
The consideration of whether what is claimed a difference makes a difference is, I think, a most effective way of determining whether some idea or theory is worth any interest or concern. If it makes no difference to how we live, then plainly we are best advised to devote real time and effort to something that does. Something that makes no difference is also something that cannot be tested or observed, and in that sense will always be speculative.
Pragmatism's focus on consequences, on effects, is one of its most attractive aspects to me. Although such as Russell insisted that it wrongly confuses what is true with what works, that seems to me a very simple-minded criticism. It isn't a question of proposing that what works is true. It's more an insistence that unless something has effects or consequences, i.e. makes a difference, it is not something which can be made use of, tested and judged as good or bad, right or wrong, useful or useless. Most of all, it should not and cannot reasonably be used for any purpose. Those who seize upon such things in making decisions make decisions for no good reason.
So it may well be that William James wrote philosophy like a novelist, and his brother Henry wrote novels like a philosopher. I can't speak to Henry James, though, as I've never been able to read him. I get the same impression reading him as I get when I read Henry Adams--the overwhelming impression of artifice, contrivance. It may have been the style of the time.
However, William James must be respected as someone who, with Peirce and Dewey and others, sought to make philosophy pertinent to how we live by judging it by its effects on how we live and disregarding it when it has no effects.